1. Field of the Invention
The invention relates to a thermoplastic polyurethane product with improved high temperature softening point, hot oil fuel resistance, and improved compression set.
2. Description of the Prior Art
Processes for the continuous production of thermoplastic polyurethanes in double shaft screw extruders with self-cleaning screws by the reaction of relatively high molecular weight polyhydroxyl compounds, polyisocyanates and chain lengthening agents have been disclosed in U.S. Pat. No. 3,963,679. In these processes, the reaction mixture must be mixed vigorously inside the screw extruder with the aid of kneading elements at a stage in which the melt still has a low viscosity (about 20 to 70 Pascals) so as to produce homogeneity in the end product. According to one variation of these processes, aggregates such as thermoplasts may be mixed with the product in the screw extruder during or after the reaction but there is no indication in these publications that the mechanical properties of thermoplasts could be improved.
U.S. Pat. No. 4,261,946, issued to Goyert et al. describes a process for the addition of polyurethane forming components to a molten thermoplastic polymer to form so-called "hard elements" in situ. In this way, Goyert is able to form very rigid and highly elastic materials from inexpensive thermoplastic materials that would otherwise have only moderate hardness and strength. The Goyert reference does not disclose a method for producing a thermoplastic polyurethane material having a high temperature softening point, resistance to thermal attack and similar characteristics which would be useful in numerous applications such as tubes, hoses, cable jacketing, film, sheet or any other extrusion profile form.